Bug#979180: Adding ZRTP

[ael]

Although the rtp library is missing from testing,
https://github.com/wernerd/ZRTPCPP provides the file(s).
So

cmake .. -DWITH_ALSA=On -DWITH_SPEEX=On -DWITH_ILBC=Off -DWITH_ZRTP=On 
-DWITH_G729=Off -DWITH_QT5=On

now works.